Offset printing press system are typically subject to many variations and defects caused by changes in ink rheology, ink-water balance, temperature, etc. These variations and defects cause continuous changes in the colors within the print during the printing process. In light of the current trend of reduced print order size coupled with increased quality demands of customers, a highly skilled pressman is needed to run the press.
Critical control is required, for example, in the setting of each of the ink keys on an offset printing press. Each ink key is adjusted both before and during the printing process so as to properly meter the amount of ink that flows onto the printing plate. In older manually operated presses, a pressman visually scans the printing plate and estimates the amount of ink needed within each of the sections controlled by the ink keys. In other systems, an optical scanner is used to scan a printing plate to determine the amount of ink needed. This information is then processed to automatically set the ink keys.
More modern presses use electromechanical means to set the ink keys remotely and to sense the actual position of the ink key actuators. The ink key data is displayed at a control table used by the pressman to effect control over each ink key. Typically, the ink keys are preset either in accordance with the pressman's judgment or by automatic means. Once the initial adjustments are made, the press is started. Further adjustments are made to the ink keys to compensate for registration of various colors, water fountains, etc. in order to improve the quality of the output until acceptable quality is achieved. As the press continues to run, further fine adjustments are made by the pressman until, usually after several hours of running, high grade printing, i.e., `OK printing,` is achieved.
The disadvantage of this system is that adjustments to the ink keys during the running of the press must be made manually by the pressman. Although, the ink keys may be remotely actuated via a control table, the adjustments are still determined by the pressman's subjective judgment.
Thus, it is desirable to have a color measurement system capable of automatic color evaluation and control similar to that performed by a skilled pressman. Such a system would help to both maintain high printing standards and to reduce printing costs.